Giant Nixie Tube Clock

This is the first clock I completed, built using giant Z568M Nixie
tubes. The digits are about 2" high; the tubes' glass envelopes
are about 4" tall overall.

(Click images for larger photos)

The clock is run by a PIC16C84 microcontroller. I probably could have
saved a few supporting components by using a 28-pin or 40-pin MCU, but I
had a few of the 16C84s around already, and at the time, the 28-pin MCUs
were only available in OTP or CERDIP versions.

Display data is sent out as serial BCD to a series of 74HC594 shift
registers. Their parallel output goes to a set of 7441 nixie tube drivers.

In addition to the BCD data, four of the shift register outputs are used to
control a set of transistors to switch the tube plates. This allows
blanking individual digits for leading-zero blanking, dimming at night
to prolong tube life, and flashing the digits for a "cursor" effect
when setting the time.

The power line frequency is used as a timebase. The power supply is on a
seperate small board. The logic supply is regulated by a LM340T5
regulator. The Nixie plate supply is regulated by an emitter-follower
biased by a 180V zener diode.

The unregulated plate supply is provided by using a dual-primary
transformer in a somewhat unconventional manner: Line voltage is fed to
one primary; the other primary (isolated from the line) is connected as a
secondary to feed a voltage doubler for the plate supply. The normal
secondary is used for the low-voltage supply.